There is a need for near infrared (NIR) sensitive silver materials, for both heat processed silver halide and conventional wet processed silver halide constructions, which produce a hard copy from various solid state output devices emitting radiation in the near infrared. For example, silver halide materials sensitized to the red and NIR may be used to produce hard copy from computer stored graphics data via a scanning laser diode source. The wavelengths of interest are primarily in the region 650 to 900 nm.
The silver imaging material requires two types of dye absorbing in the near infrared. One dye must be present in the silver halide emulsion in order to sensitize the silver halide to wavelengths in the NIR. The second dye must be an infrared dye which may be required in a coating separate from the emulsion layer in order to improve image quality. This second dye is known as an antihalation dye and is required to prevent internal reflection of scattered light which would otherwise expose the silver halide emulsion adjacent to the point of intended exposure. Such a dye may also be incorporated in the silver emulsion layer to absorb scattered light and improve the acutance of the image.
Dyes in the antihalation coatings of films sensitive to visible or near infrared radiation must be stable during coating, storage and exposure of the film, but are required to bleach during the processing of the film. The resulting decomposition products should leave essentially no residual stain or near-U.V. absorption in the film. It is known that mono-,tri-and pentamethine oxonol dyes have been useful in antihalation layers and as filter dyes in conventional silver halide formulations sensitive to the visible region. However, the known examples of these classes of dyes do not have suitably long wavelength absorption for use in near infrared sensitive materials. Heptamethine oxonols are known to absorb at longer wavelengths, but very few examples have been prepared and characterised, owing to the instability of the compounds concerned. For example, British Patent No. 506385 discloses such compounds, but does not described their synthesis. In "Rodd's Chemistry of Carbon Compounds" (ed. S. Coffey, 2nd edition, vol. IVB p.140, Elsevier 1977), no data is recorded for heptamethine oxonols. Fabian and Hartman, in "Light Absorption of Organic Colourants" (Springer Verlag, 1980) p.174, state that a few oxonols of chain length greater than 5 have been prepared, but no details are given, except that the compounds are unstable. Grossel et al (Electrochimica Acta Vol. 34 p. 425, 1989) disclose electrochemical data for one heptamethine oxonol dye along with several others of shorter chain length. Generally, these materials of the prior art do not show sufficient stability for photographic use.
A class of heptamethine oxonol dyes has now been found which have particular utility as antihalation dyes in red and NIR sensitive photographic elements.